Power supply units of this type in the known art are represented by the boost type and the buck-boost type. A typical boost type power supply unit is provided with a basic circuit structure that includes a rectifying circuit and a smoothing capacitor with a choke coil connected between them. A switching element is connected between power lines that are on a rear stage of the choke coil and a diode is connected in series with a power supply line that is on a rear stage of the switching element. Furthermore, an output capacitor is connected between the power lines on the rear stage of the diode and both ends of the output capacitor are led to an output terminal. In this circuit structure, the output capacitor is charged in one direction with the energy supplied by the power source and the energy stored in the choke coil while the switching element is ON via the diode to generate boosted voltages at both ends of the output capacitor.
A typical example of the buck-boost type power supply unit is a flyback converter system in which a switching element is connected in series with the input winding of the transformer for conversion and the energy stored in the transformer for conversion while the switching element is ON is transferred to the load side during the subsequent OFF period. However, the power supply units in the known art described above present the following problems.
First, in a boost type power supply unit, there is only a switching element connected between the power lines and there is no switching element to be connected in series to the power lines. Because of this, when the switching element has stopped its switching operation and is in an OFF state, the input voltage is output through the lines without regulation. Furthermore, if the output terminals are shorted or overloaded, protection against over current cannot be implemented. In addition, when the input voltage is applied, a rush current runs to the smoothing capacitor, and it may cause electrical damage to the rectifying circuit.
Another problem is that, since the output voltage is always higher than the input voltage, a DC-DC converter with high allowable input voltage is required on the rear stage. As for a buck-boost type power supply unit, since it is provided with a transformer for conversion which isolates the input terminals from the output terminals, unlike a boost type power supply unit, it does not present problems caused by the absence of a switching element connected in series to the power supply line. However, in this case, the entire energy must be transferred via the transformer for conversion. This, in turn, leads to lowered efficiency in energy conversion and a larger circuit.
In order to solve these problems, for example, JP H8-70,573A (Prior Art 1) discloses a power supply unit having a first energy transfer circuit and a second energy transfer circuit. The first energy transfer circuit, which includes a transformer and a first switching element, provides a configuration similar to the buck-boost type power supply unit and the second energy transfer circuit, which includes a second switching element and the output winding of said transformer, provides a configuration similar to the boost type power supply unit, thereby solving the above-mentioned respective problems of the boost type power supply unit and the buck-boost type power supply unit.
However, since the power supply unit of Prior Art 1 has a control system based upon only the output voltage value, there is a period in which the current through the transformer is zero. Consequently, in large power supply (200 W or larger, for example) to the load, the transformer current runs only in a period in which the transformer current is not zero. As a result, the peak value of the transformer current rises up.
Thus, in large power supply to the load, the power supply unit of Prior Art 1 requires a large-sized transformer for large current flowing or a large-capacity switching element for large current switching. Which poses problems such as cost increase and size increase in power supply units.